Resonant tunneling diodes comprise semiconductor structures having two large band-gap barrier layers with a single low band-gap quantum well between them. Collector 30 and emitter 31 contact regions are provided in the semiconductor structure to provide collection and supply of electrons as illustrated in the simplified band diagram of FIG. 1. The thicknesses of barrier layers 34 and 35 and the thickness of the quantum well 36 between them, and the composition of these structures, are chosen so that quantum effects create a single resonant energy level 37 slightly above the emitter conduction band edge 38. As the emitter 31 is negatively biased, the two bands will come into alignment at a peak voltage V.sub.p, as illustrated in FIG. 2, and electrons will tunnel through to the collector region 30 where they are collected. As the negative bias is increased still further, the emitter conduction band 38 rises above the resonant energy level 37, as illustrated in FIG. 3, drastically reducing the tunneling current. The result is a negative resistance region that creates the utility of the resonant tunneling diode (RTD). As the emitter bias is increased still further, current will rise again as electrons are emitted over the barrier. FIG. 4 shows a typical current-voltage relationship curve 40 for a typical RTD.
The high speed voltage transition occurs when the RTD is switched from the stable point "a" in FIG. 4 to the stable point "b". The voltage swing is maximized by making the voltage V.sub.p low and the voltage V.sub.v high. Switching speed is maximized by making the peak current I.sub.p as large as possible for a given conduction area. The valley current at the voltage V.sub.v is of great significance for high-speed applications, and should be as low as possible to maximize the current available to charge the load capacitance thus reducing the switching time.
It is desirable that RTDs be highly reliable and stable over time and with temperature changes, and have typical performance characteristics that include voltage swings of one to two volts, peak current densities of 100 to 200 kA/cm.sup.2, peak currents of 10 to 20 mA, peak voltages of 1 volt, peak to valley current ratios of at least 3, and a rise time of less than 2 picoseconds. Such characteristics have been achieved previously in devices made of pseudomorphic AlAs/InAs/AlAs quantum wells fed by lattice-matched InGaAs contact layers, and grown on InP by molecular beam epitaxy. However, the InP material system is not well suited for practical applications, and the technology is immature. GaAs would be ideal, but GaAs/AlAs RTDs cannot reach the performances of InP-based devices.